1. Field of the Invention
The present invention relates to an improved method and apparatus for automated detection of events which are either hazardous and/or have important consequences for the drilling or production process of hydrocarbon reservoirs and in particular to provide rapid generation of real-time alarms with a low false alarm rate. Specifically it pertains to a drilling information system which interprets a range of surface and downhole measurements so as to detect hazardous events during the drilling process.
2. Description of the Related Art
Over the last few years, an important improvement to drilling rig instrumentation has been the introduction of "intelligent" systems, which automatically monitor key measurements on the rig and give early, real-time indications of events which are either hazardous and/or have important consequences for the drilling process. These events include, but are not limited to, the early detection of a kick, or influx from the formation being drilled, pipe washouts, fluid loss from the well being drilled and sticking pipe.
Kicks are traditionally detected by monitoring pit volumes and by comparison of flow-in and flow-out of the well being drilled. Monitoring trends in delta-flow has been shown to be particularly successful for early influx detection, as detailed in "Delta Flow: An Accurate, Reliable System for Detecting Kicks and Loss of Circulation During Drilling", J. M. Speers and G. F. Gehrig, SPE/IADC 13496.
Conventional rig instrumentation relies on low-resolution gauges which prevent trends from being readily identified. A simple alarm can be raised when the flow out or the pit volume exceeds a preset value, but to avoid continuous false alarms the preset value is generally set high, allowing small influxes to go unnoticed.
While existing computer systems can provide much more sensitive and reliable indicators during the early stages of events such as kicks, there are still many limitations.
For example, previous computerized systems have typically used the Hinkley algorithm as detailed in "Inference About the Change-Point from Cumulative Sum Tests", D. V. Hinkley, Biometrika 42(6), pp 1897-1908, 1971 in order to detect trends in the data channel(s) being monitored. The algorithm is, however, optimized for the detection of step changes. The algorithm is used to detect a linear trend by approximating the trend as a sequence of steps. This technique is therefore not optimal for the detection of events such as kicks and washouts as these have functional forms which are poorly approximated by a sequence of step changes. In particular, when using the known approach, the operator still needs to set a sensitivity parameter to correctly balance the trade-off between the detector's sensitivity and the number of false alarms. While it is easy to reduce the sensitivity of the systems if false alarms occur, it is less obvious when to increase the sensitivity again, unless an event is already in progress. The best sensitivity setting varies depending on measurement noise, which may be caused by rig motion on a floating rig or large cuttings interfering with correct operation of the flow sensor. The operation of the system therefore needs to be closely monitored..
A method for modeling the probability of a drill string becoming stuck is described in the U.S. Pat. No. 5,508,915. In this method, a canonical point representation of the drilling process is derived from borehole measurements and drilling parameters. The point is mapped into a canonical space to indicate the probability of sticking.
Further there are known mathematical methods, such as the Bayesian theory, which allow to discern different hypotheses when given experimental evidence (data). The Bayesian theory has been attributed to Rev. Thomas Bayes, who first discovered its principles back in 1763. A modern summary of Bayesian theory is presented for example by E. T. Jaynes, in an article titled "Confidence Intervals versus Bayesian Intervals", which is published in: "Papers on Probability, Statistics and Statistical Physics", R. D. Rosenkrantz (Ed.), Kluwer, 1983, pp. 149-209.
In view of the above cited prior art it is an object of the invention to provide an alarm system to detect events with greater sensitivity than known systems. It is a particular object of the invention to provide such a system for which requires less or no human intervention for setting sensitivity or threshold values.